Thucydides was born an aristocrat and while his sympathies were largely oligarchic, Pericles' ability to curb the excesses of democracy impressed him. However, he was scathing of Pericles' successors, whose demagoguery he felt led to endless quarelling and prevarication in the assembly and hamstrung the Athenian war effort.

In contrast to Herodotus, an earlier Greek historian, Thucydides favoured an objective political and military account of the war, rather than a moralising attempt to draw ethical principles or appeal to divine justice. He sought primary evidence wherever possible, and was himself involved in many of the incidents he describes. He tells us that he was a general during the war, survived the Athenian plague and was exiled from the city due to a military failure. It is because he was an exile of Athens that he was able to travel among the Peloponnesian allies. This meant that he could view the war from the perspective of both sides. His materialist 'scientific' approach to history was revolutionary and the cause-and-effect relationships he identified in the Peloponnesian War were what led him to believe that his history would be instructive down the ages.

Summarised from Robin Lane Fox, The Classical World: An Epic History of Greece and Rome (2005), 165-168.
See also www.crystalinks.com/thucydides.html